The acoustic properties of wood are well documented. The selection of wood as a construction material, particularly for acoustic applications such as instruments and concert halls, is important because the sound is produced by the vibrations of the material itself. The characteristics which determine the acoustic performance of a material are density, Young's modulus and loss coefficient (see Wegst, U. 2006. Wood for sound. American Journal of Botany 93: 1439-1448).
Pitch, loudness and timbre represent the three auditory attributes of sound. Pitch represents the perceived fundamental frequency of a sound, which can be precisely determined through physical measurement. The intensity of a sound is a function (the square of the amplitude) of the vibration of the originating source. In addition to a pitch associated with a sound, an acoustic body such as a musical instrument also has a pitch which is audible when vibrated. The acoustics of a given body depend on shape as well as the material from which the body is made (Wegst).
It is known that the sound quality of stringed instruments is enhanced with age, specifically from actual playing-time (or use). The wood used to construct the instruments provides a more pleasing result as the instrument is played. It is for this reason that such a high value is placed on vintage instruments.
The vibration associated with use of the instrument causes subtle changes in the pliability of the wood. Vibration alters the natural resins within the wood. Moreover, finishes such as lacquer, commonly applied to wooden stringed instruments, are affected by vibration and result in the loss of plasticizers. These changes usually take many years.
Others have sought to shorten the time needed to gain the desired effects of aging. For example, U.S. Pat. No. 2,911,872 describes a motor powered apparatus which mechanically bows the strings of a violin. The system can be set up such that the strings can be played at any selected position and bowed in succession. U.S. Pat. No. 5,031,501 describes a small shaker board device which is attached to the sound board of a stringed instrument. The shaker is then driven by a musical signal to simulate what the sound board experiences as it is being played. These approaches both provide automatic means to simulate playing the instrument, thus allowing the instrument to be aged without the expenditure of time or effort by a real musician. However, both approaches take a prolonged period of time to age a new instrument because they basically simulate playing the instrument; aging occurs in real time.
U.S. Pat. No. 5,537,908 developed a process for wooden stringed instruments that utilizes broadband vibration from a large electromagnetic shaker and controller. The instrument is attached to a specially designed shaker fixture and then subjected to broadband vibration excitation. The broadband input provides excitation over the frequency range of 20 to 2,000 Hz, providing accelerated aging compared to single tone inputs from earlier methods. Experienced musicians attested to hearing improvement in sound producing ability after application of this method. In addition, simple vibration measurements showed an increase in instrument response. The process, however, requires direct contact or coupling with a large electromagnetic shaker which can and result in damage to the instruments processed. In addition, the upper frequency limit of such shakers is about 2,000 Hz, whereas the full audible spectrum is from 20 up to 20,000 Hz.
In addition to its use in the construction of instrument, wood is an important component in the acoustic makeup of structures. Concert halls in particular are meticulously constructed to maximize acoustic effect. To this end, great care goes into the selection and placement of construction materials. Two important factors, with regard to room acoustics, are reverberation time as well as the level of reverberant sound. Wood is often used to maximize acoustic effect through the placement of wooden panels which act as reflectors and resonators, and the use of wood flooring and stage construction are necessary for the optimization of the sound field and reverberation time (Wegst, 2006).
An acoustic system such as a musical instrument or a concert hall, possesses acoustic resonances. Resonance refers to the tendency of a system to oscillate at maximum amplitude at certain frequencies, known as the system's resonance frequencies (or resonant frequencies). At these frequencies, even small periodic driving forces produce large amplitude vibrations, because the system stores vibrational energy.
Acoustic resonance is the tendency of the acoustic system to absorb more energy when the frequency of its oscillations matches the system's natural frequency of vibration (its resonance or resonant frequency) than it does at other frequencies. Most objects have more than one resonance frequency, especially at harmonics of the strongest resonance. An acoustic system will easily vibrate at the strongest frequencies, and vibrate to a lesser degree at other frequencies. Materials, such as wood, possess the ability to react to its particular resonance frequency even when it is subjected to a complex excitation, such as an impulse or a wideband noise excitation. The net effect is a filtering-out of all frequencies other than its resonance.